Recently, wind turbines have received increased attention as an environmentally safe and relatively inexpensive alternative energy source. With this growing interest, considerable efforts have been made to develop wind turbines that are reliable and efficient.
Generally, wind turbines use the wind to generate electricity. The wind turns multiple blades connected to a rotor. The spin of the blades caused by the wind spins a shaft of the rotor, which connects to a generator that generates electricity. Specifically, the rotor is mounted within a housing or nacelle, which is positioned on top of a truss or tubular tower. Utility grade wind turbines, i.e., wind turbines designed to provide electrical power to a utility grid can have relatively large rotors ranging to thirty or more meters in diameter. Blades on these rotors transform wind energy into a rotational torque or force that drives one or more generators that, in some instances, are coupled to the rotor through a gearbox. The gearbox may be used to step up the inherently low rotational speed of the turbine rotor for the generator to efficiently convert mechanical energy to electrical energy, which is provided to a utility grid.
Some turbines utilize generators that are directly coupled to the rotor without using a gearbox. In these instances, doubly fed induction generators (DFIG) may be used. Power converters are used to transfer the power for the wound rotor of the generator to a grid connection. In operation, a required level of energy will pass through a DC link of the power converter. Under certain conditions (e.g., transient power conditions), a high power mismatch between the rotor and the grid connection temporally exists and voltage transients become amplified such that a DC link voltage level can increase above normal allowed or rated levels.
Known systems for absorbing or deflecting power during excessive power level conditions include using a fast acting shorting means, e.g., a crowbar circuit, between the rotor terminals of the doubly fed induction generator and the rotor converter. In operation, these shorting devices provide a short circuit at the rotor terminals, for example, during the excessive power level conditions, to prevent excess power flowing to the rotor converter. Excess power can result in the development of an excess DC link voltage that can damage the converter and halt the operation of the wind turbine system.
The known extra shorting devices not only add cost to the wind turbine system, but may cause high torque peaks to the generator shaft torque that excite vibrations in the coupled drive train of the wind turbine. Excessive forces must be accounted for where sudden changes of extracted power cause transient or oscillatory forces to exceed rated force or the force related displacement to exceed the rated displacement.
While the cost of providing increased strength for these components is high, the capability provided by such components can be used to increase their value. On the other hand, limits that must be placed on the operating system may reduce this value. The need for the system to provide current for grid stability may reduce the generators capability to provide frequency support where a portion of the grid stability current is supplied by the generator alone.
Thus, there is a need for a method and system to dependably supply the grid stability reactive power currents while allowing the generator to provide the frequency supporting current without need to strengthen all the turbine equipment.